ABSTRACT Plasticity is a hallmark feature of the neural system controlling breathing. One well-studied form of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a prolonged increase in phrenic activity triggered by acute intermittent hypoxia (AIH). Multiple distinct cellular mechanisms contribute to AIH-induced pLTF, depending on the severity of hypoxic episodes. Whereas the Q pathway requires 5-HT2 receptor activation on phrenic motor neurons, the S pathway requires adenosine 2A receptor activation. These distinct intra-cellular signaling pathways interact via powerful cross-talk inhibition; indeed, concurrent pathway activation actually cancels pLTF expression. Although we have learned a great deal about intra-cellular signaling mechanisms of AIH-induced pLTF, we know little concerning the role of inter-cellular signaling. Recent reports demonstrate that glia regulate neuroplasticity in multiple neural systems, including microglia, the innate immune cells of the CNS. Since virtually nothing is known concerning the role of microglia in regulating AIH-induced phrenic motor plasticity, our primary goal is to explore this knowledge gap in normal rats and in rats with systemic inflammation. The fundamental hypothesis guiding our proposal is that microglia differentially regulate competing pLTF mechanisms elicited by moderate versus severe AIH (Aim 1). We propose a unified model to explain such differential microglial regulation of AIH-induced pLTF. During severe hypoxia, we propose that phrenic motor neurons release Fractalkine (a chemokine unique to neurons), activating microglial Fractalkine receptors (unique to microglia) and triggering the microglial adenosine release necessary for severe AIH-induced pLTF (Aim 2). With moderate AIH, diminished inter-cellular Fractalkine and adenosine signaling permit the expression of serotonin-dependent pLTF (ie. Q pathway), but with a persistent adenosine constraint (Aim 3). We further propose that even mild systemic inflammation enhances microglial adenosine release during moderate AIH, increasing cross-talk inhibition and suppressing pLTF expression (Aim 4). Finally, since AIH-induced pLTF exhibits a profound age-dependent sexual dimorphism, we will test the hypothesis that phrenic motor neuron- microglia interactions are differentially affected by age in female versus male rats (Aim 5). This project will be the first attempt to identify a specific role of microglia in any form of respiratory motor plasticity, greatly increasing our mechanistic understanding concerning the importance of inter-cellular signaling in respiratory motor plasticity. Since repetitive AIH is emerging as a novel therapeutic intervention to restore breathing (and other movements) in people with debilitating disorders such as cervical spinal injury or ALS, greater understanding of factors regulating AIH-induced plasticity will help optimize AIH protocols and improve chances for successful translation of this promising therapeutic modality. Increased understanding of age and sex effects will establish ?ground rules? for translation to clinical disorders that afflict both men and women.